To perform filtrations of solutions (fluids or gases containing suspended solids) bearing multiple forms of solids having different particle sizes and solubility constants, and/or complex solids, filtration systems, particularly those that employ vacuum assisted filtration, use a rather laborious and time consuming manual procedure of introducing a solution into a filter cup having a porous barrier, i.e., a filter, to retain particles of a desired size (solute) and allow passage of a solvent (filtrate) component. Often times, the desired solid cannot be separated and isolated without the use of multiple filtration operations that may require a multitude of reagents to break down the solution components and multiple filter media to obtain the desired solid.
With conventional gravimetric, or vacuum-assisted filtration systems, a series of filters have to be used to achieve the desired result. For example, as shown in U.S. Pat. No. 5,695,639, a modular filter system is shown that includes a spout member designed to direct a solvent into a container. A top portion of the spout is configured to receive in releasable engagement, a reservoir element, or modular filter cup.
To secure a filter to the apparatus, the filter is sandwiched between the tight fitting, but releasable reservoir and spout. To perform a multi-step filtration process with this apparatus, each filtration operation requires the reservoir and spout to be disassembled and the filter carefully removed and replaced with a different filter for the next step. If sterility of the solute is important to the procedure, a sterile implement, e.g., forceps, may have to be used to remove the filter.
The fluid portion that flows via the spout into a container, e.g., a flask, is reintroduced into the reservoir to begin another filtration cycle. The steps are repeated serially until the desired solid(s) has been separated out of the solution. The process is slow and cumbersome due to the structure of the filter cup/filter/stem assembly.
In U.S. Pat. No. 4,301,010, a multi-component vacuum filter funnel is disclosed that solves the problem of solids migrating around the perimeter of filters placed in filter cups. The apparatus includes a vacuum intake member (similar to a filter cup stem) that creates a seal between the filter cup assembly and a container, such as a flask. An optional funnel having a spout is placed within the intake member and forms an airtight seal with the intake member. The spout extends beyond a bottom outlet of the intake member. An upper funnel member (filter cup) having a perforated plate formed on a bottom has threading on an outer lower end that engages threading on an upper inner perimeter edge of the intake member.
A filter is placed on the perforated plate to perform the filtration function. A threaded cylindrical sleeve member has threading formed around an outside bottom edge of the member that engages corresponding threading on a lower end of an inner wall of the upper funnel member. The engaged features of the sleeve member and upper funnel member secure the filter firmly against the perforated plate, particularly about the filter's perimeter edge. This configuration ensures a tight seal of the filter's edge to prohibit the migration of solids around the filter's edge.
While solving the problem of solute migration around the filter, this configuration is enormously cumbersome when a multi-step filtration process is required. Each change of a filter requires the sleeve member to be rotated out of the upper funnel member and the upper funnel member to be rotated out of the intake member to enable retrieval of the filter. This assembly and disassembly process has to be repeated for each successive filter.
As gravimetric and vacuum-assisted filtration processes often require multiple filtration steps, achieving this by multiple exchanges of filter cups with changed filters may cause additional problems with respect to solute loss and material contamination. Each problem can adversely affect quantitative as well as qualitative analyses.
The process of stacking conventional filter cups is equally problematic as the registered walls of stacked cups result in too much surface area contact. With the addition of vacuum assisted filtration, the binding of nested cups is increased making separation potentially very difficult due to the negative pressure coupled with likely capillary action of fluid migrating into the interface of adjacent cups.
What is needed and desired is a filter cup apparatus that simplifies and streamlines multi-step filtration processes including pre-filtration and final filtration steps by providing a filter train to prevent or eliminate solute loss and contamination, and that provides a means to facilitate separation of nested cups to retrieve desired solutes. These and other objects of the invention will become apparent from a reading of the following summary and detailed description of the invention.